These USB-TTL converters with PL2303 chips are crap. I've got many problems with these sticks, i.e. one described here. The problem lies in PL2303 chip - 99,9% of these terribly cheap sticks have counterfeited ICs. They may look like original ones from Prolific, but obviously they do not. Differences are very subtle, for example mine have invalid production code.
As Artūras Jonkus said You should use stick with original chip, these from FTDI are great (but not cheap), and there is a small chance for buying counterfeited FTDI chips. Original Prolific chips are also very good, but there are plenty fake ones on the market. Also good USB-TTL chips without many fakes are from Cypress.